1. Field of the Invention
The present invention relates to an improvement on the method of removing, through chemical etching, a damaged layer that is generated on the surface of a monocrystalline silicon wafer during a process of producing the wafer.
2. Description of the Related Art
A conventional process of producing a mirror-polished semiconductor wafer typically comprises the steps of slicing a monocrystalline ingot of silicon or the like to obtain a semiconductor wafer; and chamfering, lapping, acid etching, mirror-polishing, and cleaning the sliced semiconductor wafer. Depending on the required specifications, the sequence of steps is changed; some steps are repeated a plurality of times; or other steps such as heat treatment and grinding are added to or replace the above-described steps. Thus, a variety of kinds of steps are performed in accordance with the specifications.
Among the above-described steps, acid etching is performed for the purpose of removing a surface damaged layer introduced in the course of mechanical machining steps such as slicing, chamfering, and lapping. In the acid etching step, the surface of a wafer is etched to a depth of a few to a few tens of microns through use of mixed acid aqueous solution composed of hydrofluoric acid, nitric acid, acetic acid, and water. However, acid etching involves the following problems:
1) The flatness of a wafer after lapping--which is indicated by thickness variation represented by, for example, TTV (Total Thickness Variation) (.mu.m) or LTV.sub.max (Local Thickness Variation) (.mu.m)--deteriorates as the etching amount increases. PA1 2) A waviness of a millimeter order or an uneven region called "peel" is generated on an etched surface. PA1 3) Harmful NO.sub.x is generated due to etching. PA1 a) Flatness established by lapping is maintained after etching. PA1 b) Generation of harmful gas is suppressed. PA1 i) If foreign mater enters pits locally existing on an etched surface and having a depth of a few microns and a diameter of a few to twelve or thirteen microns, the foreign matter causes generation of particles and/or contamination in a subsequent step. PA1 ii) Since deep pits exist and surface roughness (Ra) increases, a polishing stock removal in a subsequent step of mirror-polishing (mechano-chemical polishing) must be increased. PA1 iii) Since an etched surface has a sharp uneven shape compared to a surface etched through acid etching, the unevenness itself serves as a source of particles.
In consideration of these problems, alkali etching is used in some cases.
Alkali etching has the following advantages:
However, alkali etching has the following disadvantages:
Accordingly, particles generated in a subsequent step and a polishing stock removal in a mirror-polishing step can be decreased if etching treatment can be performed while flatness attained through lapping is maintained, so as to remove a mechanically formed damage layer, improve the surface roughness, efficiently decrease the depth of deep pits locally formed due to the etching, and smooth the uneven shape of the surface.